thats representative of the job, so it should be represented in the interview. open book interviews test for resource finding and implementation skills.
The main problem with your response is that the threshold for "basic concept" is arbitrary by framework, by language, by time, by interviewer. Revert back to interviewing things relevant for the day to day job, look at what developers actually employed there did over the last few weeks, and interview on that.
Asking about async/sync in JS is completely different from asking someone what obscure features like finalizers are in Java. One is used on an hourly basis when writing code, the other one is never used except in obscure legacy code.
If you do what you're proposing and just ask about projects I think you'll find it's easy for incompetent people who are floundering around and copy pasting from stackoverflow without understanding what they're copying to pass your bar. I copy from stack overflow all the time but I understand what I'm copying and why it works and when it doesn't work.
`async` questions are the most day job relevant questions I can imagine for a javascript developer. It is so pervasive, a such a simple piece of syntactic sugar over basic asynchronicity, that not knowing it is damning along a number of axes.
You can certainly stumble your way to a semi-working app without knowing that, but your mental model of how Javascript works is fundamentally lacking if you don't know that.
and its also not my point about "async in JS", thats a symptom in a field where ever random person pulled in to conduct an interview has their own random threshold of competence, when simply conducting an interview that mimics the day to day on the job would be more accurate
"did you google that in 10 seconds" great!
"did the compiler autocomplete your method name or show you a list of likely possibilities?" great!
"did you post a question on a QA website, while you went to work on other parts of the task while the hints and answers rolled in?" efficient!
"did your compiler show you a syntax error immediately that you corrected immediately?" great!
"did you compile it as you went and rapidly iterate towards the write answer like everyone else working here?" perfect!
whiteboarding, and most collaborative coding platforms, fail to replicate 9/10ths of that. so it tells nothing about the candidate.